Holborn Head Quarry

[ND 080 710]

Highlights

Holborn Head Quarry in Caithness (Highland) has produced specimens of 10 or 11 species of fossil fishes, and it is especially well known for the fine, abundant specimens of the small osteolepid Osteolepis panderi.

Introduction

In the large disused cliff-top quarry at Ness of Litter, 3 km W of Holburn Head (Figure 6.25), the flags have been quarried down to tough non-fissile siltstones, and the extensive sloping floor of the quarry lies along a single flat bedding plane. This is a common feature of several flagstone quarries, which were worked down bed by bed for paving flags, without the use of explosives. Crampton and Carruthers (1914, pl. 7) showed the quarry in operation early in the 20th century. Trewin and Hurst (1993) have provided an excursion guide. The beds dip at a shallow angle of about 5° to the north-north-east. The taphonomy of these fish beds was recently investigated by Hamilton and Trewin (1994).

Description

The quarry exposes a 6 m high face of carbonate or organic laminated siltstones with subaqueous shrinkage cracks. These are lacustrine sediments from the Ham–Skarfskerry Subgroup of the Upper Caithness Flagstone Group (Donovan et al., 1974; Westoll, in House et al., 1977; Hamilton and Trewin, 1994). The flagstones are thinner-bedded and with lighter weathering colours than those of the underlying Latheron Subgroup, as seen at Banniskirk Quarry (q.v.). Grey and buff sandstones occur in the upper part of the Subgroup. The rocks are of probable mid-Givetian age (Westoll, in House et al., 1977), as indicated by the fauna that probably belongs to Fish Zone 5 of Donovan et al. (1974), as found also at Weydale Quarry (q.v.).

The fish-bearing stratum is 130 mm below the bedding plane which forms the floor of the quarry, and is exposed by a small excavation in the south-east corner of the quarry, near the entrance. The fishes are in a tough, dark brown ish grey, calcareous siltstone laminite, and they are difficult to remove. A thickness of 260 mm of fish-bearing siltstones are exposed in the excavation, and the base of the unit is not seen. Fishes are very common throughout the 260 mm thick unit, and they are usually complete, but flattened and poorly preserved. Small attractive complete osteolepids, Osteolepis panderi, are very common. Disarticulated skull roofs can also be found and, although less common than complete specimens, their preservation is better. An important addition to knowledge of these strata and their fossil fish is Hamilton and Trewin's (1994) detailed consideration of the taphonomy and palaeoecology.

There are problems in assigning provenances of older museum specimens from this area since the locality label 'Holburn Head' includes several sites of different age, from Brims Hill in the west to the headland of Holburn Head. 'Thurso' is also often used very loosely as a specimen label (Miles and Westoll, 1963), and can include material from Holburn Head Quarry. Therefore, in the list below, some of the rarer species are queried, and may not be from this site at all.

Fauna

The fish fauna from Holburn Head Quarry includes the remains of active forms most of which were common and widespread throughout the Orcadian Basin.

Acanthodii: Acanthodiformes: Acanthodidae

Mesacanthus peachi Egerton, 1861

M. pusillus (Agassiz, 1844)

?Cheiracanthus sp.

acanthodian indet.

Placodermi: Arthrodira: Coccosteidae

Dickosteus threiplandi Miles and Westoll, 1963

Placodermi: Arthrodira: Homosteidae

?Homosteus milleri Traquair, 1888

Osteichthyes: Sarcopterygii: Osteolepiformes: Osteolepididae

Osteolepis panderi Pander, 1860

Gyroptychius agassizi Traill, 1841

Thursius pholidotus Traquair, 1888

Osteichthyes: Sarcopterygii: Porolepiformes:

Holoptychiidae

Glyptolepis sp.

Osteichthyes: Sarcopterygii: Dipnoi: Dipteridae

Dipterus cf. valenciennesi Sedgwick and Murchison, 1828

Dickosteus threiplandi is a large coccosteid fish with a total length (head plus body) of over 0.5 m. The holotype is from Spittal (q.v.), but a large complete specimen from 'Holburn Head' was described by Miles and Westoll (1963), and specimens of plates from 'Brims' were also figured.

Osteolepis panderi (syn. O. microlepidotus Valenciennes and Pentland, 1860) is by far the most common fish at Holburn Head. Hamilton and Trewin (1994) recorded the distribution and abundance of this and four other species throughout the most productive 50 cm of the fish bed at Holborn Head Quarry, and also the size distribution of O. panderi within the bed (Figure 6.26). Prior to its complete description by Jarvik (1948a), it had always been known as O. microlepidotus, but Jarvik determined that Pander had, in fact, been the first to describe it. Jarvik did not erect a lectotype in his redescription, because Pander's original material in the Imperial Academy of Science, St Petersburg, was unavailable immediately following World War II. Instead, Jarvik (1948a) used a mass of material from 'Thurso' and other Caithness sites. Osteolepis panderi is a small species, usually not exceeding 135 mm in length, and therefore considerably smaller than most O. macrolepidotus, a close relative from Caithness and Orkney. There are also differences in the pattern of head bones, with numerous small bones at the front of the headshield in O. macrolepidotus, and very slight subdivision in this area in O. panderi. Osteolepis macrolepidotus is restricted to the Achanarras horizon (Saxon, 1978), whereas O. panderi is found in the Ham–Skarfskerry Subgroup, and rarely in the Mey Subgroup, in the Thurso area and in Weydale Quarry (q.v.), and in the upper Stromness and Rousay Groups of Orkney.

Interpretation

As at other localities, the vertebrate-bearing rocks are interpreted as lacustrine deposits. Fish fossils are found in a dark-coloured siltstone, which may represent an anoxic phase or a salinity crisis leading to a high fish mortality; mass mortality levels are associated with dolomitic laminae and hence with salinity crises (Hamilton and Trewin, 1994). The dominance of Osteolepis panderi may be a reflection of the absence of a large predator, and the lack of juveniles of this species suggests that the young kept to other parts of the lake and so were spared the mass mortality event.

Hamilton and Trewin (1994) plotted the distribution and abundance of the fish within the Holborn Head Quarry fish bed. This band is some 50 cm of dense carbonate or organic laminite, deposited apparently during the deepest part of the lake cycle (Figure 6.26).

Conclusion

The conservation value of Holburn Head Quarry results from the production of excellent specimens of the small osteolepiform Osteolepis panderi, as well as a fauna of acanthodians and placoderms typical of Fish Bed 4. Finds have been made recently in an excavation in the quarry floor to reach the fish bed, and there is future potential for more finds.

References